Gas Sensing Nonlinear Interferometry

The increase in greenhouse gas molecules such as carbon dioxide and methane are causing global climate change, including rapid changes in temperature which threaten to increase sea levels and
destabilise global ecosystems. It is therefore of paramount importance that we have effective techniques of monitoring greenhouse gases in the atmosphere and finding causes of rapid increases,
such as gas leaks from pipelines or decomposing waste. Many of these molecules have fundamental vibrational resonances at mid-infrared (MIR) wavelengths, however detection at such wavelengths
is poor: MIR detectors are expensive, require cooling, and when compared to Silicon detectors they generally have much lower pixel counts and can be several orders of magnitude noisier.
Previous work has been done using up-conversion to convert MIR photons to silicon detectable wavelengths for imaging, however this requires large powers to counter the low conversion efficiency, which is nontrivial and makes the method unsuitable for photosensitive samples. Instead, we can consider a nonlinear interferometer (NLI) based on the principle of induced coherence to circumvent both of these problems - we pump a nonlinear crystal and a non-degenerate spontaneous parametric down-conversion (SPDC) process generates photon pairs, with the signal photons at a wavelength readily detected by Silicon, and the idler photons resonant with the vibrational resonances of the target gas. The idler photons are interfaced with the target, before all three paths are passed back through the nonlinear crystal and SPDC occurs once more, with new photons overlapping and interfering with the previous ones. Any modulation on the idler photons from the target creates distinguishability, which can be observed in the interference visibility of the signal photons, allowing for imaging and sensing of the target without detecting the idler photons.